If you've seen a poster tube, you'll know what I mean. They're quite solid; enough to fit survive the rigours of a first stage (I hope. Flying Hypersonic with Cardboard? Sounds like something John Powell would do ). Well, take one of those, and outfit it with lightweight plastic tanks which can hold Butane, pressurized and cooled into a Liquid. Use a solid Oxidiser such as SO3. Attach rockets to thew tip, to pull the craft forward. To simplify use, the engines should be pressure fed. The engines could be made out of Steel.

Use an Balloon/Airship to lift the whole Booster to a height of maybe 30km, and fire the rockets

Your on the right track there buddy! You need to consider that once you get to space you have to accelerate to obital velosity! 17,000mph and that's pretty hard to do! You need to think about how your going to guide this thing into an orbital trajectory as well. The governments that be will want to know exactly where your going! in other words you need a flight plan. Keep plugging and ask questions there are answers.

If you've seen a poster tube, you'll know what I mean. They're quite solid; enough to fit survive the rigours of a first stage (I hope. Flying Hypersonic with Cardboard? Sounds like something John Powell would do ). Well, take one of those, and outfit it with lightweight plastic tanks which can hold Butane, pressurized and cooled into a Liquid. Use a solid Oxidiser such as SO3. Attach rockets to thew tip, to pull the craft forward. To simplify use, the engines should be pressure fed. The engines could be made out of Steel.Use an Balloon/Airship to lift the whole Booster to a height of maybe 30km, and fire the rockets

I like your thinking Terraformer - lets take it a step further.

1/ Cardboard propellant tank - why not. Just need to line the inside with a barrier, could be the following.a/ Layer of Al foil which might allow you to hold cyrogenic propellants or NOX, as paper is good insulator.b/ Get some HDPE film and layer inside would be compatible with peroxide, or rotomold HDPE inside your tube.

2/ Form the ends by standard hand lay-up of fiber glass, or cast them from resin/fiber matrix. Clue them into place and fill around them with epoxy, this method is used for disposable SRMs so shouldn't be an issue. I have clued nozzles and head caps into motors operating at over 700psi before.

3/ Hoop wrap the outside with glass or carbon / epoxy you could do this by hand. Or could use a product like bi-axial sleeve to reinforce your axial strength.

Or you could skip the liner if your propellant is storable and house it in a bladder like a wine case. Say you went for a Propane/ Hydrogen Peroxide bi-prop, you could house both propellants in separate bladders in the same tank. With the expanding Propane bladder acting like a piston to expel the peroxide as well, once the Peroxide is all used you could vent your Propane for either spin up thrusters or stage separation.Or even just a little extra delta-v..

Isp of Propane/ 85% Peroxide at altitude is nothing to sneeze at either (<300sec), with very low pressure (130psi tank) or could go Butane (75psi tank) and still get similar performance.

I don't know about the inverse hybrid you have proposed, there has been very little actual testing done on inverse hybrids, so you would blazing trail as such.

For your motor you could go for tribrid using a cardboard combustion chamber as fuel, and your Peroxide/ Propane liquids. You get some ablative cooling here as bonus, probably just need wrap the outside with some composite/ high temp epoxy. Will be very light weight...

Or just go full solid. It's easier, which balances out the disadvantages of the lower Isp, but is't as controllable. Oh well, this is an idea for a first stage anyway. Your idea for liquid propellents could be the 2nd/3rd stage.

For ejecting the stage... would firing the motors on the next stage do that? If the stages aren't linked together completely (ie they're joined by some kind of rods instead of one being right on top of the other), could the exhaust be used to burn away the connecters?

But... carboard combustion chamber and nozzle?! I suppose the nozzle could be - if it's lauched from a high altitude the exhauust could be quite cool - but the combustion chamber? Unless it's combusted in the engine, right?

Or just go full solid. It's easier, which balances out the disadvantages of the lower Isp, but is't as controllable. Oh well, this is an idea for a first stage anyway. Your idea for liquid propellents could be the 2nd/3rd stage.

Solid propellant is good but cost is an issue.. AP, and HTPB aren't really cheap. And like I said a low pressure liquid motor, can achieve 10 to 30sec more ISP at Balloon launch altitudes.

Terraformer wrote:

For ejecting the stage... would firing the motors on the next stage do that? If the stages aren't linked together completely (ie they're joined by some kind of rods instead of one being right on top of the other), could the exhaust be used to burn away the connecters?

Yes all are relevant methods, but its important to understand the tip off dynamics when your stages separate to ensure your flight angle isn't effected. i.e such as your 2nd stage nozzle making contact with your 1stage as it falls away. A simple thing but many early space launches failed due to staging errors, even space-x hit this one. Being able to use up your left over Propane or Butane to effectively pop your stages apart, would help add to your total delta-V. You could simply pressurize the coupler between your stages, and perhaps use shear pins that snap under the force holding. Same method has been used on large model rockets using CO2 system to blow off the nose cone, for recovery purposes.

Terraformer wrote:

But... carboard combustion chamber and nozzle?! I suppose the nozzle could be - if it's lauched from a high altitude the exhauust could be quite cool - but the combustion chamber? Unless it's combusted in the engine, right?

Very thick cardboard tubes have been used for solid propellant motor cases for 100's of years, and as I said paper (cellulose) is an okay hybrid fuel too. I did mention though putting one wrap of composite over outside to ensure strength. On the nozzle part I didn't suggest you make your throat from cardboard, just the chamber. I would be looking to mold something from phenolic resin filled with silica as a throat.